The performance and mission applications of future ramjet and scramjet powered vehicles are highly dependent on protecting the engines and airframe from high heat loads encountered at hypersonic speeds. As aircraft flight speeds increase to the high supersonic and hypersonic regimes, aerodynamic heating becomes increasingly severe and critical demands are placed on the structural and thermal capabilities of the engines and airframe. At flight speeds near Mach 4, the air taken on board these vehicles will be too hot to cool the engines and airframe. Therefore, it will probably be necessary to use the fuel as the primary coolant. To simplify fuel storage and handling considerations, any fuel chosen for this role should have handling and storage characteristics similar to those found in conventional aircraft turbine fuels.
Turbine fuels themselves have long been used as coolants on high performance aircraft because of their capacity to absorb sensible and latent heat. Sensible heat is the heat required to heat the fuel to its boiling point. Latent heat is the heat required to vaporize the fuel. The capacity to absorb sensible and latent heat is referred to as the fuel's physical heat sink. The use of turbine fuels and other conventional liquid hydrocarbon fuels as physical heat sinks, however, is generally limited to moderate temperature applications to avoid fouling the aircraft's cooling or fuel injection systems with deposits formed by fuel decomposition. As a result, these fuels may not be appropriate physical heat sinks for high speed vehicles in which relatively high temperatures will be encountered.
Cryogenic fuels, such as liquid methane and liquid hydrogen, have a sufficient physical heat sink for cooling high speed vehicles and do not present the problems of deposit formation and system fouling. However, they have drawbacks which may render them impractical to use. First, such fuels have a low density, which means large tank volumes, hence large vehicles, are required to hold sufficient fuel. Second, the need to maintain the fuels at cryogenic temperatures presents formidable logistics and safety problems, both on the ground and during flight, especially as compared to conventional aircraft turbine fuels.
An alternate approach would be to use endothermic fuels to provide the needed engine and airframe cooling. Endothermic fuels are fuels which have the capacity to absorb large quantities of physical and chemical heat. Like the turbine and cryogenic fuels discussed above, endothermic fuels are capable of absorbing sensible and latent heat and, therefore, have a physical heat sink. In addition, endothermic fuels are capable of absorbing a heat of reaction to initiate an endothermic decomposition reaction. The capacity to absorb a heat of reaction is referred to as the fuel's chemical heat sink. By combining the physical and chemical heat sinks of an endothermic fuel, the fuel is capable of absorbing two to four times as much heat as fuels which are used only as physical heat sinks and up to twenty times more heat than conventional turbine fuels that are limited to moderate temperatures by their propensity to decompose and form deposits. Furthermore, endothermic fuels offer storage and handling advantages over cryogenic fuels because they are liquids under ambient conditions on the ground and at high altitudes, and have higher densities than cryogenic fuels.
Most work with endothermic fuels has been limited to the selective dehydrogenation of naphthenes, such as methylcyclohexane (MCH), on precious metal catalysts. The decomposition of MCH to toluene and hydrogen over a platinum on alumina catalyst has been demonstrated to provide a chemical heat sink of about 900 Btu/lb, nearly as much as the MCH's physical heat sink of about 1000 Btu/lb. However, the total heat sink of about 1900 Btu/lb may not be adequate to provide the cooling required for very high speed vehicles. Moreover, the cycle life of the platinum/alumina catalyst is apt to be fairly short when operated at the required severe conditions. The MCH must be exceptionally pure because the platinum catalyst is susceptible to sulfur, halide, metals, and particulate poisoning. However, pure MCH has a much lower flash point and much higher vapor pressure than conventional aircraft turbine fuels, necessitating special handling and storage considerations. In addition, the toluene produced by decomposing MCH is a poor fuel for high speed engines because it produces soot during combustion. Soot causes excessive radiative heating of combustor liners and turbine blades, and leads to undesirable visible and infrared emissions.
Accordingly, what is needed in the art is a method of cooling high speed vehicles using an endothermic fuel which provides a high total heat sink, yields products with superior combustion characteristics, does not require precious metal catalysts, and which has handling and storage characteristics similar to those of conventional aircraft turbine fuels.